It is known that the improvement of energy productivity is one of the main question in photovoltaic systems which are both stand-alone and connected to an electric energy distribution network (or grid connected). In this context, MPPT technique plays an important role since, when correctly designed, it allows to maximise the output power of the photovoltaic field by continuously tracking the maximum power point that depends on the temperature of the photovoltaic modules and on the solar radiation conditions, as described by S. Liu and R. A Dougal in “Dynamic multiphysics model for solar array”, IEEE Transactions on Energy Conversion, Vol. 17, No. 2, pp. 285-294, June 2002.
The MPPT technique has been treated in many different ways in the prior art. In this regard, examples of implementation in fuzzy logic, with neural networks, with pilot cells, and based on digital signal processors (or DSP) have been proposed by T. Noguchi, S. Togashi and R. Nakamoto in “Short-current pulse-based maximum-power-point tracking method for multiple photovoltaic-and converter module system”, IEEE Trans. Ind. Electron., vol. 49, no. 1, pp. 217-223, February 2002, by C. Hua, J. Lin, and C. Shen in “Implementation of a DSP-controlled photovoltaic system with peak power tracking”, IEEE Trans. Ind. Electron., vol. 45, no. 1, pp. 99-107, February 1998, by N. Mutoh, M. Ohno, and T. Inoue in “A method for MPPT control while searching for parameters corresponding to weather conditions for PV generation systems”, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1055-1065, June 2006, by N. Mutoh and T. Inoue in “A control method to charge series-connected ultraelectric double-layer capacitors suitable for photovoltaic generation systems combining MPPT control method”, IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 374-383, February 2007, by T.-F. Wu, C.-H. Chang, and Y.-H. Chen in “A fuzzy-logic-controlled single-stage converter for PV-powered lighting system applications”, IEEE Trans. Ind. Electron., vol. 47, no. 2, pp. 287-296, April 2000, by M. Veerachary, T. Senjyu, and K. Uezato in “Neural-network-based maximum-power-point tracking of coupled-inductor interleaved-boost converter-supplied PV system using fuzzy controller”, IEEE Trans. Ind. Electron., vol. 50, no. 4, pp. 749-758, August 2003, by I. S. Kim, M. B. Kim, and M. J. Youn in “New maximum power point tracker using sliding-mode observer for estimation of solar array current in the grid-connected photovoltaic system”, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1027-1035, June 2006, by E. Roman, R. Alonso, P. Ibanez, S. Elorduizapatarietxe, and D. Goitia in “Intelligent PV module for grid-connected PV systems”, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1066-1073, June 2006, by J. M. Kwon, K. H. Nam, and B. H. Kwon in “Photovoltaic power conditioning system with line connection”, IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1048-1054, June 2006, by W. Xiao, W. G. Dunford, P. R. Palmer, and A. Capel in “Application of centered differentiation and steepest descent to maximum power point tracking”, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp. 2539-2549, October 2007, by R. Gules, J. De Pellegrin Pacheco, H. L. Hey, and J. Imhoff: in “A Maximum Power Point Tracking System With Parallel Connection for PV Stand-Alone Applications”, IEEE Trans. on Industrial Electronics, Vol. 55, No. 7, July 2008, by M. Veerachary, T. Senjyu, and K. Uezato in “Voltage-based maximum power point tracking control of PV system”, IEEE Trans. Aerosp. Electron. Syst., vol. 38, no. 1, pp. 262-270, January 2002, and by E. Koutroulis, K. Kalaitzakis, and N. Voulgaris in “Development of a microcontroller-based, photovoltaic maximum power point tracking control system”, IEEE Trans. Power Electron., vol. 16, no. 1, pp. 46-54, January 2001. Also less complex implementations based on the Perturb and Observe technique are widely used, thanks to the fact that, if correctly designed, they may lead to particularly high values of MPPT efficiency, as described by N. Femia, G. Petrone, G. Spagnuolo, and M. Vitelli in “Optimization of perturb and observe maximum power point tracking method”, IEEE Trans. Power Electron., vol. 20, no. 4, pp. 963-973, July 2005, and by N. Femia, D. Granozio, G. Petrone, G. Spagnuolo, and M. Vitelli in “A predictive and adaptive MPPT perturb and observe method”, IEEE Trans. Aerosp. Electron. Syst., vol. 43, no. 3, pp. 934-950, July 2007, J. Y. Ahn et al., in “Dual-module based maximum power point tracking control of PV system”, IEEE Applied Power Electronics Conference and Expostion, Vol. 3, pages 1509-1514, 22 Feb. 2004, [XP010704038] discloses a MPPT method for controlling a dual module PV system wherein two modules, each including a section of photovoltaic field connected to a respective power converter, operate in parallel.
However, any MPPT technique specification presents some particular limitations, mainly due to the complexity of the circuit implementation.
In fact, the latter (almost always based on digital electronics) requires devices for measuring the photovoltaic power, such to allow the execution of operations of multiplication between voltages and currents, and a significant number of sensing devices for sensing many circuit electrical quantities, in particular voltages and currents. Such detections are affected by ineliminable noise components (due to the high frequency switching operation and/or to an inadequate filtering), the overall combination of which causes tracking (based on noisy measures) to be affected by a significant noise as well that renders it not completely precise. Consequently, such techniques may make the system operate in a non maximum power point or, sometimes, they may lead to unstable oscillations of the system, as described by Petrone, G. Spagnuolo, R. Teodorescu, M. Veerachary, and M. Vitelli in “Reliability Issues in Photovoltaic Power Processing Systems”, IEEE Trans. on Industrial Electronics, Vol. 55, No. 7, pp. 2569-2580, June 2008.
Moreover, the aforementioned complex circuit implementations often require the exclusive use of a specific type of power converters.
Also, in so called grid connected applications, 100 Hz disturbances coming from the network are capable to cause failure of the MPPT techniques (as it occurs for instance for the Perturb and Observe technique).